There seem to be some Christian assumptions in your assertion that religion 'started to gain momentum a couple of thousand years ago'. However, Christianity is a relative newcomer as far as religions are concerned. Judaism goes much further back. The Egyptians were living in a society ruled by religious beliefs at least 2000 years before the birth of Christ. Other religions go back even further. Religion (in its various forms) had gained full momentum well before Christianity was born.
Most religions seem to embrace the idea of an 'afterlife' which, in turn, usually involves some concept of 'heaven'.
The ancient Greeks recognised the concept of Hades but, as I understand it, they didn't regard it in the same way that many modern religions see Hell. (i.e. a place which receives the souls of the damned).
The idea of the souls of the dead being consigned to either Heaven or Hell seems to be primarily associated with the three religions which recognise Abraham as a patriarch. i.e. Judaism, Christianity and Islam. Since Abraham probably lived around 2000 BC, that suggests that the idea of 'heaven and hell' has been around for at least 4000 years.
Theoretically, it wouldn't be hard to get an estimate for the total number of people consigned to both Heaven and Hell, by simply calculating the total number people who've ever lived. However, such a calculation presents a problem for anyone who accepts the truth of evolution but who also believes that only humans (and not apes) have souls. The question they must struggle to answer relates to determining the point at which soul-less apes evolved into humans with a soul.
This might help:
http://en.wikipedia.org/wiki/Hell
Chris